The main server file mixed portable session handling with Unix details:
sending SIGHUP to end a session, decoding exec.ExitError, the euid check
for whether the process can switch users, agent forwarding's chown of the
socket, and reading /etc/ssh host keys as root. Those now sit behind small
functions (hangupProcess, waitProcess, canSwitchToLocalUser,
handleSSHAgentForwarding, systemHostKeyFile, isRootUser) in the new
process_unix.go, along with the incubator's forwarded-environment pipe
helpers, and the session's *exec.Cmd moves into an embedded osSessionState
struct defined there, so that the portable code no longer refers to the
process representation at all.
user.go keeps only the portable userMeta and userLookup; the login shell
and default PATH logic moves to user_unix.go. The SFTP child entrypoint and
its stdio adapter, which incubator.go and incubator_plan9.go each had a
copy of, move to sftp.go. The c2n usernames handler gains a hook for
platforms that list users some other way than /etc/passwd.
The agent socket's uid and gid are parsed as 31-bit rather than 32-bit
unsigned values so that the conversion to int for os.Chown cannot
overflow on 32-bit platforms, which is the pattern CodeQL flags.
Updates #cleanup
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I4c7e2b9a0d3f5e1c8b6a4d2f0e9c7b5a3d1f8e6c
